Radar, as used in collision avoidance, weather, and reconnaissance, is well known. It is believed that in the near future radar will additionally be utilized on trucks and automobiles for intelligent cruise control, collision avoidance, and other navigational functions.
However, once radar is implemented on a wide scale for such purposes, the potential for mutual interference among nearby radar sets becomes substantial.
In co-located radar systems, i.e., wherein both the receiving and transmitting antenna share a common location, signals from other, nearby transceivers may potentially interfere substantially with one another unless specific methods are utilized so as to mitigate such interference.
Typically, a directional antenna is used so as to reduce the potential for such interference by greatly attenuating those signals which are not received within the directional antenna's main beam. However, the potential for interference substantially increases when an interfering radar transceiver is located within the antenna's main beam.
As such, it is beneficial to provide means for mitigating interference among proximate radar sets when the positioning of one radar set within the main beam of another radar set cannot be avoided. This situation would be prevalent on roadways wherein automobiles would frequently be disposed ahead of each other's forward looking radar sets.
According to prior art methodology, orthogonal signals have been used in an attempt to mitigate such interference. The use of such orthogonal signals ideally provides a means for selecting a desired signal while rejecting co-resident (within the radar antenna's main beam), undesirable signals.
However, due to the limited number of such orthogonal signals, as discussed below, reliable use depends upon assuring that no two proximate radar sets utilize a common set of orthogonal signals. When such radar sets are independently operated (not responsive to a common controller), it is difficult to assure that each proximate radar set utilizes a unique orthogonal signal, so as not to cause such interference.
Orthogonal signals are defined herein to include signals constructed from a set of functions that cross-correlate to a small value and auto-correlate to a large value. This property facilitates the use of a matched filter (or correlation detector) to recognize one desired signal and attenuate all other signals within the function set. Thus, a radar set may easily discriminate between its own radar signal and potentially interfering proximate radar signals which are radiate into the radar set's main antenna beam.
The use of sinusoidal signals of differing frequencies comprises the most common use of orthogonal signals in radar. Matched filters, typically approximated via the use of simple bandpass filters, pass the desired frequency sinusoidal signal and reject sinusoidal signals of different frequencies.
It is well known in the art that several other orthogonal basis sets can be used for radar signals, as well. An example of some of these basis sets is provided below:
1. Sinusoids with differing frequencies;
2. Pulsed sinusoids with differing pulse repetition frequencies;
3. Pulsed sinusoids with pseudo-random pulse repetition frequencies;
4. Sinusoids with pseudo-random phase coding;
5. Sinusoids with pseudo-random frequency coding;
6. Walsh functions;
6. Cross-polarization; and
7. Various combinations of Items 1-6.
Radar systems that employ the use of such signal/matched filters from one of the aforementioned basis sets tend to minimize interference from other signals, particularly within the same basis set. However, in practice, due to bandwidth limitations or for other reasons, all basis sets only have a finite number of useable signals. Thus, individual functions or waveforms within a given basis set need to be reused when the number of required signals exceeds the finite number of useable signals.
Thus, when many radar sets are operated within close proximity such as in automobile applications, the potential exists for having another radar within a particular radar antenna's main beam which utilizes the same signal function, thus resulting in undesirable interference.
As such, it is beneficial to provide means for mitigating interference among radars sets utilizing orthogonal signal sets and directional antennae.